Plasticizing system and rubber composition for tyre containing said system

ABSTRACT

Plasticizing system which can be used in particular for the plasticizing of a tyre diene elastomer composition comprising, in combination:
         a plasticizing hydrocarbon resin, the glass transition temperature of which is greater than 0° C.; and   a tri- or pyromellitate ester corresponding to the formula (I):       

     
       
         
         
             
             
         
       
         
         
           
             in which the R radicals, which are identical or different, represent a hydrocarbon radical and the R′ radical represents hydrogen or the COOR group. 
           
         
       
    
     The invention also relates to a rubber composition incorporating the said plasticizing system and to the use of such a composition for the manufacture of a tyre or tyre semi-finished product made of rubber, in particular of a tyre tread, exhibiting an improved wear resistance without having a detrimental affect on its wet grip.

The present invention relates to rubber compositions intended inparticular for the manufacture of tyre treads; it relates moreparticularly to plasticizing systems which can be used for theplasticizing of such compositions.

As is known, a tyre tread has to meet a large number of oftenconflicting technical requirements, including a low rolling resistance,a high wear resistance and a high grip on both the dry road and the wetroad.

This compromise in properties, in particular from the viewpoint of therolling resistance and the wear resistance, was able to be improved inrecent years with regard to energy-saving “Green Tyres”, intended inparticular for passenger vehicles, by virtue of the use of novel weaklyhysteretic rubber compositions having the characteristic of beingreinforced predominantly by specific inorganic fillers described asreinforcing, in particular by highly dispersible silicas (HDS), capableof rivalling, from the viewpoint of the reinforcing power, conventionaltyre-grade carbon blacks. Thus, today, these reinforcing inorganicfillers are gradually replacing carbon blacks in tyre treads, all themore so as they have another known virtue, that of increasing the gripof tyres on wet, snowy or icy roads.

The improvement in the grip/wear resistance compromise remains, however,a constant preoccupation of designers of tyres, whether the lattercomprise treads comprising silica or carbon black as filler.

The use is known, to promote the properties of wear and abrasionresistance and/or of grip of tyre treads, of plasticizing hydrocarbonresins in their rubber compositions, as described, for example, inPatents or Patent Applications U.S. Pat. No. 3,927,144, GB-A-2 178 046,JP-A-61-190538, JP-A-09-328577, WO-A-91/18947, WO-A-02/088238,WO-A-02/072688 and WO-A-02/072689.

Improvements to the above solutions have been obtained by combiningthese plasticizing hydrocarbon resins with certain specific plasticizingagents.

In particular, in order to further increase the wear and abrasionresistance of tyre treads, plasticizing systems have been provided whichcomprise, in combination, non-aromatic oils of the MES or TDAE type withterpene hydrocarbon resins, such as polylimonene, or with C₅fraction/vinylaromatic copolymer hydrocarbon resins (see ApplicationsWO-A-2005/087859 and WO-A-2006/061064).

In point of fact, on continuing their research, the Applicant Companieshave discovered a novel plasticizing system based on a plasticizinghydrocarbon resin which makes it possible, in comparison with acombination of a hydrocarbon resin and of an IVIES oil, to furtherimprove the wear resistance of tyres without having a detrimental affecton their wet grip.

Thus, a first subject-matter of the invention is a rubber compositioncomprising at least one diene elastomer, one reinforcing filler and oneplasticizing system, characterized in that the said plasticizing systemcomprises, in combination:

-   -   a plasticizing hydrocarbon resin, the glass transition        temperature (Tg) of which is greater than 0° C.; and    -   a tri- or pyromellitate ester corresponding to the formula (I):

-   -   in which the R radicals, which are identical or different,        represent a hydrocarbon radical and the R′ radical represents        hydrogen or the COOR group.

The invention also relates, per se, to a plasticizing system which canbe used for the plasticizing of a diene rubber composition, the saidsystem comprising, in combination, a plasticizing hydrocarbon resin, theTg of which is greater than 0° C., and a compound of formula (I), andalso to the use of such a system for the plasticizing of a tyre dienerubber composition.

Another subject-matter of the invention is a process for preparing arubber composition exhibiting an improved wear resistance, thiscomposition being based on a diene elastomer, on a reinforcing fillerand on a plasticizing system, the said process comprising the followingstages:

-   -   incorporating in a diene elastomer, in a mixer:        -   a reinforcing filler;        -   a plasticizing system;    -   everything being kneaded thermomechanically, in one or more        goes, until a maximum temperature of between 110° C. and 190° C.        is reached;    -   cooling the combined mixture to a temperature of less than 100°        C.;    -   subsequently incorporating:        -   a crosslinking system;    -   kneading everything up to a maximum temperature of less than        110° C.;    -   extruding or calendering the rubber composition thus obtained;        and being characterized in that the plasticizing system        comprises at least:    -   one plasticizing hydrocarbon resin, the glass transition        temperature (Tg) of which is greater than 0° C.; and    -   a tri- or pyromellitate ester corresponding to the above founula        (I).

Another subject-matter of the invention is the use of a compositionaccording to the invention for the manufacture of a finished article orof a semi-finished product made of rubber intended for anyground-contact motor vehicle system, such as tyre, internal safetysupport for a tyre, wheel, rubber spring, elastomeric joint, othersuspension element and vibration damper.

A particular subject-matter of the invention is the use of a compositionaccording to the invention for the manufacture of tyres or semi-finishedproducts made of rubber intended for these tyres, these semi-finishedproducts preferably being chosen from the group consisting of treads,crown reinforcing plies, sidewalls, carcass reinforcing plies, beads,protectors, underlayers, rubber blocks and other internal rubbers, inparticular decoupling rubbers, intended to provide the bonding or theinterface between the abovementioned regions of the tyres.

A more particular subject-matter of the invention is the use of acomposition according to the invention for the manufacture of a tyretread.

Another subject-matter of the invention is the tyres themselves and thesemi-finished products, in particular tyre treads, when they comprise arubber composition in accordance with the invention.

The tyres of the invention are particularly intended to equip motorvehicles, such as passenger vehicles, SUVs (Sport Utility Vehicles),two-wheel vehicles (in particular motor cycles), aeroplanes, such asindustrial vehicles chosen from vans, heavy-duty vehicles—that is tosay, underground, bus, heavy road transport vehicles (lorries, tractors,trailers) or off-road vehicles, such as heavy agricultural vehicles orearthmoving equipment—, and other transportation or handling vehicles.

The invention and its advantages will be easily understood in the lightof the description and exemplary embodiments which follow.

I.—MEASUREMENTS AND TESTS

The rubber compositions are characterized, before and after curing, asindicated below.

I-1. Mooney Plasticity

Use is made of an oscillating consistometer as described in FrenchStandard NF T 43-005 (1991). The Mooney plasticity measurement iscarried out according to the following principle: the composition in theraw state (i.e., before curing) is moulded in a cylindrical chamberheated to 100° C. After preheating for one minute, the rotor rotateswithin the test specimen at 2 revolutions/minute and the working torquefor maintaining this movement is measured after rotating for 4 minutes.The Mooney plasticity (ML 1+4) is expressed in “Mooney unit” (MU, with 1MU=0.83 newton.metre).

I-2. Rheometry

The measurements are carried out at 150° C. with an oscillating discrheometer, according to standard DIN 53529—part 3 (June 1983). Thechange in the rheometric torque as a function of time describes thechange in the stiffening of the composition as a result of thevulcanization reaction. The measurements are processed according tostandard DIN 53529—part 2 (March 1983): t_(i) is the induction time,that is to say the time necessary for the start of the vulcanizationreaction; t_(α) (for example t₉₀) is the time necessary to achieve aconversion of α %, that is to say α % (for example 90%) of thedifference between the minimum and maximum torques. The conversion rateconstant, denoted K (expressed as min⁻¹), which is first order,calculated between 30% and 80% conversion, which makes it possible toassess the vulcanization kinetics, is also measured.

I-3. Shore A Hardness

The Shore A hardness of the compositions after curing is assessed inaccordance with Standard ASTM D 2240-86.

I-4. Tensile Tests

These tests make it possible to determine the elasticity stresses andthe properties at break. Unless otherwise indicated, they are carriedout in accordance with French Standard NF T 46-002 of September 1988.The “nominal” secant moduli (or apparent stresses, in MPa) or the “true”secant moduli (reduced in this case to the real cross section of thetest specimen) are measured in second elongation (i.e., after a cycle ofaccommodation) at 10% elongation (denoted “M10” and “E10” respectively),100% elongation (“M100” and “E100” respectively) and 300% elongation(“M300” and “E300” respectively). All these tensile measurements arecarried out under the standard conditions of temperature (23±2° C.) andhygrometry (50±5% relative humidity) according to French Standard NF T40-101 (December 1979). The breaking stresses (in MPa) and theelongations at break (in %) are also measured, at a temperature of 23°C.

I-5. Dynamic Properties

The dynamic properties are measured on a viscosity analyser (MetravibVA4000) according to Standard ASTM D 5992-96. The response of a sampleof vulcanized composition (cylindrical test specimen with a thickness of4 mm and with a cross section of 400 mm²), subjected to a simplealternating sinusoidal shear stress, at a frequency of 10 Hz, at atemperature of 40° C., is recorded. A strain amplitude sweep is carriedout from 0.1% to 50% (outward cycle) and then from 50% to 1% (returncycle); the maximum value of the loss factor, denoted tan(δ)_(max), isrecorded for the return cycle.

I-6. Tests on Tyres

A) Rolling Resistance

The rolling resistance is measured on a test drum according to the ISO87-67 (1992) method. A value greater than that of the control,arbitrarily set at 100, indicates an improved result, that is to say alower rolling resistance.

B) Wear Resistance

The tyres are subjected to actual on-road running on a specific motorvehicle until the wear due to the running reaches the wear indicatorspositioned in the grooves of the tread. A value greater than that of thecontrol, arbitrarily set at 100, indicates an improved result, that isto say a greater mileage travelled.

C) Wet Grip

In order to assess the wet grip performances, the behaviour of the tyresfitted to a specific motor vehicle travelling, under limit speedconditions, on a circuit with a great many bends and which is sprayed soas to keep the ground wet is analysed. The minimum time necessary totravel the entire circuit is measured; a value greater than that of thecontrol, arbitrarily set at 100, indicates an improved result, that isto say a shorter journey time.

II. DETAILED DESCRIPTION OF THE INVENTION

The rubber composition according to the invention, which can be used inparticular for the manufacture of a tyre or of a tyre tread, comprisesat least one diene elastomer, one reinforcing filler and one specificplasticizing system.

Unless expressly indicated otherwise, the percentages shown in thepresent patent application are % by weight.

II-1. Diene Elastomer

The term “diene” elastomer or rubber should be understood as meaning, ina known way, an (one or more are understood) elastomer resulting atleast in part (i.e., a homopolymer or a copolymer) from diene monomers(monomers carrying two carbon-carbon double bonds which may or may notbe conjugated).

These diene elastomers can be classified into two categories:“essentially unsaturated” or “essentially saturated”. The term“essentially unsaturated” is understood to mean generally a dieneelastomer resulting at least in part from conjugated diene monomershaving a level of units of diene origin (conjugated dienes) which isgreater than 15% (mol %); thus it is that diene elastomers such as butylrubbers or copolymers of dienes and of α-olefins of EPDM type do notcome within the preceding definition and can in particular be describedas “essentially saturated” diene elastomers (low or very low level ofunits of diene origin, always less than 15%). In the category of“essentially unsaturated” diene elastomers, the term “highlyunsaturated” diene elastomer is understood to mean in particular a dieneelastomer having a level of units of diene origin (conjugated dienes)which is greater than 50%.

Given these definitions, the term diene elastomer capable of being usedin the compositions in accordance with the invention is understood moreparticularly to mean:

-   -   (a)—any homopolymer obtained by polymerization of a conjugated        diene monomer having from 4 to 12 carbon atoms;    -   (b)—any copolymer obtained by copolymerization of one or more        conjugated dienes with one another or with one or more        vinylaromatic compounds having from 8 to 20 carbon atoms;    -   (c)—a ternary copolymer obtained by copolymerization of ethylene        and of an α-olefin having 3 to 6 carbon atoms with a        non-conjugated diene monomer having from 6 to 12 carbon atoms,        such as, for example, the elastomers obtained from ethylene and        propylene with a non-conjugated diene monomer of the        abovementioned type, such as, in particular, 1,4-hexadiene,        ethylidenenorbornene or dicyclopentadiene;    -   (d)—a copolymer of isobutene and of isoprene (butyl rubber) and        also the halogenated versions, in particular chlorinated or        brominated versions, of this type of copolymer.

Although it applies to any type of diene elastomer, a person skilled inthe art of tyres will understand that the present invention ispreferably employed with essentially unsaturated diene elastomers, inparticular of the type (a) or (b) above.

The following are suitable in particular as conjugated dienes:1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-di(C₁-C₅alkyl)-1,3-butadienes, such as, for example, 2,3-dimethyl-1,3-butadiene,2,3-diethyl-1,3-butadiene, 2-methyl-3-ethyl-1,3-butadiene or2-methyl-3-isopropyl-1,3-butadiene, an aryl-1,3-butadiene,1,3-pentadiene or 2,4-hexadiene. The following, for example, aresuitable as vinylaromatic compounds: styrene, ortho-, meta- orpara-methylstyrene, the “vinyltoluene” commercial mixture,para-(tert-butyl)styrene, methoxystyrenes, chloro styrenes,vinylmesitylene, divinylbenzene or vinylnaphthalene.

The copolymers can comprise between 99% and 20% by weight of diene unitsand between 1% and 80% by weight of vinylaromatic units. The elastomerscan have any microstructure which depends on the polymerizationconditions used, in particular on the presence or absence of a modifyingand/or randomizing agent and on the amounts of modifying and/orrandomizing agent employed. The elastomers can, for example, be block,random, sequential or microsequential elastomers and can be prepared indispersion or in solution; they can be coupled and/or star-branched oralso functionalized with a coupling and/or star-branching orfunctionalization agent.

The following are suitable: polybutadienes, in particular those having acontent (molar %) of 1,2-units of between 4% and 80% or those having acontent (molar %) of cis-1,4-units of greater than 80%, polyisoprenes,butadiene/styrene copolymers and in particular those having a styrenecontent of between 5% and 50% by weight and more particularly between20% and 40%, a content (molar %) of 1,2-bonds of the butadiene part ofbetween 4% and 65% and a content (molar %) of trans-1,4-bonds of between20% and 80%, butadiene/isoprene copolymers, in particular those havingan isoprene content of between 5% and 90% by weight and a glasstransition temperature (Tg, measured according to ASTM D3418) of −40° C.to −80° C., or isoprene/styrene copolymers, in particular those having astyrene content of between 5% and 50% by weight and a Tg of between −25°C. and −50° C. In the case of butadiene/styrene/isoprene copolymers,those having a styrene content of between 5% and 50% by weight and moreparticularly of between 10% and 40%, an isoprene content of between 15%and 60% by weight and more particularly between 20% and 50%, a butadienecontent of between 5% and 50% by weight and more particularly of between20% and 40%, a content (molar %) of 1,2-units of the butadiene part ofbetween 4% and 85%, a content (molar %) of trans-1,4-units of thebutadiene part of between 6% and 80%, a content (molar %) of 1,2-plus3,4-units of the isoprene part of between 5% and 70% and a content(molar %) of trans-1,4-units of the isoprene part of between 10% and50%, and more generally any butadiene/styrene/isoprene copolymer havinga Tg of between −20° C. and −70° C., are suitable in particular.

To sum up, the diene elastomer of the composition according to theinvention is preferably chosen from the group of the highly unsaturateddiene elastomers consisting of polybutadienes (abbreviated to “BR”),synthetic polyisoprenes (IR), natural rubber (NR), butadiene copolymers,isoprene copolymers and the blends of these elastomers. Such copolymersare more preferably chosen from the group consisting ofbutadiene/styrene copolymers (SBR), isoprene/butadiene copolymers (BIR),isoprene/styrene copolymers (SIR) and isoprene/butadiene/styrenecopolymers (SBIR).

According to a specific embodiment, the diene elastomer is predominantly(that is to say, for more than 50 pce) an SBR, whether an SBR preparedin emulsion (“E-SBR”) or an SBR prepared in solution (“S-SBR”), or anSBR/BR, SBR/NR (or SBR/IR) or also BR/NR (or BR/IR) blend. In the caseof an SBR elastomer, use is made in particular of an SBR having astyrene content of between 20% and 30% by weight, a content of vinylbonds of the butadiene part of between 15% and 65%, a content (molar %)of trans-1,4-bonds of between 15% and 75% and a Tg of between −20° C.and −55° C.; such an SBR can advantageously be used as a blend with a BRpreferably having more than 90% (molar %) of cis-1,4-bonds.

According to another specific embodiment, the diene elastomer ispredominantly (for more than 50 pce) an isoprene elastomer. This is thecase in particular when the compositions of the invention are intendedto constitute, in the tyres, rubber matrices of certain treads (forexample for industrial vehicles), of crown reinforcing plies (forexample of working plies, protection plies or hooping plies), of carcassreinforcing plies, of sidewalls, of beads, of protectors, ofunderlayers, of rubber blocks and other internal rubbers providing theinterface between the abovementioned regions of the tyres.

The term “isoprene elastomer” is understood to mean, in a known way, anisoprene homopolymer or copolymer, in other words a diene elastomerchosen from the group consisting of natural rubber (NR), syntheticpolyisoprenes (IR), the various copolymers of isoprene and the blends ofthese elastomers. Mention will in particular be made, among isoprenecopolymers, of isobutene/isoprene copolymers (butyl rubber—DR),isoprene/styrene copolymers (SIR), isoprene/butadiene copolymers (BIR)or isoprene/butadiene/styrene copolymers (SBIR). This isoprene elastomeris preferably natural rubber or a synthetic cis-1,4-polyisoprene; use ispreferably made, among these synthetic polyisoprenes, of thepolyisoprenes having a level (molar %) of cis-1,4-bonds of greater than90%, more preferably still of greater than 98%.

According to another specific embodiment, in particular when it isintended for a tyre sidewall or for an airtight internal rubber of atubeless tyre (or other air-impermeable component), the composition inaccordance with the invention can comprise at least one essentiallysaturated diene elastomer, in particular at least one EPDM copolymer orone butyl rubber (optionally chlorinated or brominated), whether thesecopolymers are used alone or as a blend with highly unsaturated dieneelastomers as mentioned above, in particular NR or IR, BR or SBR.

According to another preferred embodiment of the invention, the rubbercomposition comprises a blend of a (one or more) “high Tg” dieneelastomer exhibiting a Tg of between −65° C. and −10° C. and of a (oneor more) “low Tg” diene elastomer exhibiting a Tg of between −110° C.and −80° C., more preferably between −105° C. and −90° C. The high Tgelastomer is preferably chosen from the group consisting of S-SBRs,E-SBRs, natural rubber, synthetic polyisoprenes (exhibiting a level(molar %) of cis-1,4-structures preferably of greater than 95%), BIRs,SIRs, SMRs and the blends of these elastomers. The low Tg elastomerpreferably comprises butadiene units according to a level (molar %) atleast equal to 70%; it preferably consists of a polybutadiene (BR)exhibiting a level (molar %) of cis-1,4-structures of greater than 90%.

According to another specific embodiment of the invention, the rubbercomposition comprises, for example, from 30 to 100 pce, in particularfrom 50 to 100 pce, of a high Tg elastomer as a blend with 0 to 70 pce,in particular from 0 to 50 pce, of a low Tg elastomer; according toanother example, it comprises, for the whole of the 100 pce, one or moreSBR(s) prepared in solution.

According to another specific embodiment of the invention, the dieneelastomer of the composition according to the invention comprises ablend of a BR (as low Tg elastomer) exhibiting a level (molar %) ofcis-1,4-structures of greater than 90% with an S-SBR or an E-SBR (ashigh Tg elastomer).

The compositions of the invention can comprise a single diene elastomeror a blend of several diene elastomers, it being possible for the dieneelastomer or elastomers to be used in combination with any type ofsynthetic elastomer other than a diene elastomer, indeed even withpolymers other than elastomers, for example thermoplastic polymers.

II-2. Reinforcing Filler

Use may be made of any type of reinforcing filler known for itscapabilities of reinforcing a rubber composition which can be used forthe manufacture of tyres, for example an organic filler, such as carbonblack, a reinforcing inorganic filler, such as silica, or a blend ofthese two types of filler, in particular a blend of carbon black andsilica.

All carbon blacks, in particular blacks of the HAF, ISAF or SAF type,conventionally used in tyres (“tyre-grade” blacks) are suitable ascarbon blacks. Mention will more particularly be made, among the latter,of the reinforcing carbon blacks of the 100, 200 or 300 series (ASTMgrades), such as, for example, the N115, N134, N234, N326, N330, N339,N347 or N375 blacks, or also, depending on the applications targeted,the blacks of higher series (for example, N660, N683 or N772). Thecarbon blacks might, for example, be already incorporated in theisoprene elastomer in the form of a masterbatch (see, for example,Applications WO 97/36724 or WO 99/16600).

Mention may be made, as examples of organic fillers other than carbonblacks, of the functionalized polyvinylaromatic organic fillers asdescribed in Applications WO-A-2006/069792 and WO-A-2006/069793.

The term “reinforcing inorganic filler” should be understood, in thepresent patent application, by definition, as meaning any inorganic ormineral filler, whatever its colour and its origin (natural orsynthetic), also known as “white filler”, “clear filler” or even“non-black filler”, in contrast to carbon black, capable of reinforcingby itself alone, without means other than an intermediate couplingagent, a rubber composition intended for the manufacture of tyres, inother words capable of replacing, in its reinforcing role, aconventional tyre-grade carbon black; such a filler is generallycharacterized, in a known way, by the presence of hydroxyl (—OH) groupsat its surface.

The physical state under which the reinforcing inorganic filler isprovided is not important, whether it is in the form of a powder, ofmicrobeads, of granules, of beads or any other appropriate densifiedform. Of course, the term reinforcing inorganic filler is alsounderstood to mean mixtures of different reinforcing inorganic fillers,in particular of highly dispersible siliceous and/or aluminous fillersas described below.

Mineral fillers of the siliceous type, in particular silica (SiO₂), orof the aluminous type, in particular alumina (Al₂O₃), are suitable inparticular as reinforcing inorganic fillers. The silica used can be anyreinforcing silica known to a person skilled in the art, in particularany precipitated or pyrogenic silica exhibiting a BET surface and a CTABspecific surface both of less than 450 m²/g, preferably from 30 to 400m²/g. Mention will be made, as highly dispersible (“HD”) precipitatedsilicas, for example, of the Ultrasil 7000 and Ultrasil 7005 silicasfrom Degussa, the Zeosil 1165MP, 1135MP and 1115MP silicas from Rhodia,the Hi-Sil EZ150G silica from PPG, the Zeopol 8715, 8745 and 8755silicas from Huber or the silicas with a high specific surface asdescribed in Application WO 03/16837.

When the compositions of the invention are intended for tyre treads witha low rolling resistance, the reinforcing inorganic filler used, inparticular if it is silica, preferably has a BET surface of between 45and 400 m²/g, more preferably of between 60 and 300 m²/g.

Preferably, the level of total reinforcing filler (carbon black and/orreinforcing inorganic filler, such as silica) is between 20 and 200 pce,more preferably between 30 and 150 pce, the optimum being in a known waydifferent depending on the specific applications targeted: the level ofreinforcement expected with regard to a bicycle tyre, for example, is,of course, less than that required with regard to a tyre capable ofrunning at high speed in a sustained manner, for example a motor cycletyre, a tyre for a passenger vehicle or a tyre for a utility vehicle,such as a heavy duty vehicle.

According to a preferred embodiment of the invention, use is made of areinforcing filler comprising between 30 and 150 pce, more preferablybetween 50 and 120 pce, of inorganic filler, particularly silica, andoptionally carbon black; the carbon black, when it is present, ispreferably used at a level of less than 20 pce, more preferably of lessthan 10 pce (for example between 0.1 and 10 pce).

In order to couple the reinforcing inorganic filler to the dieneelastomer, use is made, in a known way, of an at least bifunctionalcoupling agent (or bonding agent) intended to provide a satisfactoryconnection, of chemical and/or physical nature, between the inorganicfiller (surface of its particles) and the diene elastomer, in particularbifunctional organosilanes or polyorganosiloxanes.

Use is made in particular of silane polysulphides, referred to as“symmetrical” or “unsymmetrical” depending on their specific structure,as described, for example, in Applications WO03/002648 (or US2005/016651) and WO03/002649 (or US 2005/016650).

“Symmetrical” silane polysulphides corresponding to the followinggeneral formula (III):

-   -   (III) Z-A-S_(x)-A-Z, in which:    -   x is an integer from 2 to 8 (preferably from 2 to 5);    -   A is a divalent hydrocarbon radical (preferably, C₁-C₁₈ alkylene        groups or C₆-C₁₂ arylene groups, more particularly C₁-C₁₀, in        particular C₁-C₄, alkylenes, especially propylene);    -   Z corresponds to one of the formulae below:

-   -   in which:    -   the R¹ radicals, which are unsubstituted or substituted and        identical to or different from one another, represent a C₁-C₁₈        alkyl, C₅-C₁₈ cycloalkyl or C₆-C₁₈ aryl group (preferably, C₁-C₆        alkyl, cyclohexyl or phenyl groups, in particular C₁-C₄ alkyl        groups, more particularly methyl and/or ethyl),    -   the R² radicals, which are unsubstituted or substituted and        identical to or different from one another, represent a C₁-C₁₈        alkoxyl or C₅-C₁₈ cycloalkoxyl group (preferably a group chosen        from C₁-C₈ alkoxyls and C₅-C₈ cycloalkoxyls, more preferably        still a group chosen from C₁-C₄ alkoxyls, in particular methoxyl        and ethoxyl),        are suitable in particular, without the above definition being        limiting.

In the case of a mixture of alkoxysilane polysulphides corresponding tothe above formula (III), in particular the usual mixtures availablecommercially, the mean value of the “x” index is a fractional numberpreferably of between 2 and 5, more preferably in the vicinity of 4.However, the invention can also advantageously be carried out, forexample, with alkoxysilane disulphides (x=2).

Mention will more particularly be made, as examples of silanepolysulphides, ofbis((C₁-C₄)alkoxyl(C₁-C₄)alkylsilyl(C₁-C₄)alkyl)polysulphides (inparticular disulphides, trisulphides or tetrasulphides), such as, forexample, bis(3-trimethoxysilylpropyl) orbis(3-triethoxysilylpropyl)polysulphides. Use is in particular made,among these compounds, of bis(3-triethoxysilylpropyl)tetrasulphide,abbreviated to TESPT, of formula [(C₂H₅O)₃Si(CH₂)₃S₂]₂, orbis(triethoxysilylpropyl)disulphide, abbreviated to TESPD, of formula[(C₂H₅O)₃Si(CH₂)₃S]₂. Mention will also be made, as preferred examples,of bis(mono(C₁-C₄)alkoxyldi(C₁-C₄)alkylsilylpropyl)polysulphides (inparticular disulphides, trisulphides or tetrasulphides), moreparticularly bis(monoethoxydimethylsilylpropyl)tetrasulphide, asdescribed in Patent Application WO 02/083782 (or US 2004/132880).

Mention will in particular be made, as coupling agent other thanalkoxysilane polysulphide, of bifunctional POSs (polyorganosiloxanes) orof hydroxysilane polysulphides (R²═OH in the above formula III), such asdescribed in Patent Applications WO 02/30939 (or U.S. Pat. No.6,774,255) and WO 02/31041 (or US 2004/051210).

In the rubber compositions in accordance with the invention, the contentof coupling agent is preferably between 4 and 12 pce, more preferablybetween 3 and 8 pce.

A person skilled in the art will understand that a reinforcing filler ofanother nature, in particular organic nature, might be used as fillerequivalent to the reinforcing inorganic filler described in the presentsection, provided that this reinforcing filler is covered with aninorganic layer, such as silica, or else comprises, at its surface,functional sites, in particular hydroxyls, requiring the use of acoupling agent in order to form the connection between the filler andthe elastomer.

II-3. Plasticizing System

The rubber compositions of the invention have the essentialcharacteristic of using a plasticizing system comprising, incombination, a plasticizing hydrocarbon resin, the Tg of which isgreater than 0° C., and a tri- or pyromellitate ester corresponding tothe formula (I), as explained in detail below.

II-3-A. Plasticizing Hydrocarbon Resin

In a way known to a person skilled in the art, the name “plasticizingresin” is reserved in the present patent application, by definition, fora compound which is, on the one hand, solid at ambient temperature (23°C.) (in contrast to the liquid plasticizing compound, such as an oil)and, on the other hand, compatible (that is to say, miscible at thelevel used, typically of greater than 5 pce) with the rubber compositionfor which it is intended, so as to act as a true diluting agent.

Hydrocarbon resins are polymers well known to a person skilled in theart which are thus miscible by nature in diene elastomer compositions,when they are additionally described as being “plasticizing”.

They have been widely described in the patents or patent applicationsmentioned in the introduction to the present document and also, forexample, in the work entitled “Hydrocarbon Resins” by R. Mildenberg, M.Zander and G. Collin (New York, VCH, 1997, ISBN 3-527-28617-9), chapter5 of which is devoted to their applications, in particular in the tyrerubber field (5.5. “Rubber Tires and Mechanical Goods”).

They can be aliphatic or aromatic or also of the aliphatic/aromatictype, that is to say based on aliphatic and/or aromatic monomers. Theycan be natural or synthetic and may or may not be oil-based (if such isthe case, also known under the name of petroleum resins). They arepreferably exclusively hydrocarbon, that is to say that they compriseonly carbon and hydrogen atoms.

Preferably, the plasticizing hydrocarbon resin exhibits at least one,more preferably all, of the following characteristics:

-   -   a Tg of greater than 20° C.;    -   a number-average molecular weight (Mn) of between 400 and 2000        g/mol;    -   a polydispersity index (PI) of less than 3 (reminder: PI=Mw/Mn        with Mw the weight-average molecular weight).

More preferably, this plasticizing hydrocarbon resin exhibits at leastone, more preferably still to all, of the following characteristics:

-   -   a Tg of greater than 30° C.;    -   a weight Mn of between 500 and 1500 g/mol;    -   a PI of less than 2.

The glass transition temperature Tg is measured in a known way by DSC(Differential Scanning Calorimetry) according to Standard ASTM D3418(1999).

The macrostructure (Mw, Mn and PI) of the hydrocarbon resin isdetermined by size exclusion chromatography (SEC): solventtetrahydrofuran; temperature 35° C.; concentration 1 g/l; flow rate 1ml/min; solution filtered through a filter with a porosity of 0.45 μmbefore injection; Moore calibration with polystyrene standards; set of 3“Waters” columns in series (“Styragel” HR4E, HR1 and HR0.5); detectionby differential refractometer (“Waters 2410”) and its associatedoperating software (“Waters Empower”).

According to a particularly preferred embodiment, the plasticizinghydrocarbon resin is chosen from the group consisting of cyclopentadiene(abbreviated to CPD) or dicyclopentadiene (abbreviated to DCPD)homopolymer or copolymer resins, terpene homopolymer or copolymerresins, C₅ fraction homopolymer or copolymer resins and the blends ofthese resins.

Use is preferably made, among the above copolymer resins, of thosechosen from the group consisting of (D)CPD/vinylaromatic copolymerresins, (D)CPD/terpene copolymer resins, (D)CPD/C₅ fraction copolymerresins, terpene/vinylaromatic copolymer resins, C₅fraction/vinylaromatic copolymer resins and the blends of these resins.

The term “terpene” combines here, in a known way, the α-pinene, β-pineneand limonene monomers; use is preferably made of a limonene monomer,which compound exists, in a known way, in the form of three possibleisomers: L-limonene (laevorotatory enantiomer), D-limonene(dextrorotatory enantiomer) or else dipentene, the racemate of thedextrorotatory and laevorotatory enantiomers.

Styrene, α-methylstyrene, ortho-, meta- or para-methylstyrene,vinyltoluene, para-(tert-butyl)styrene, methoxystyrenes, chlorostyrenes,le vinylmesitylene, divinylbenzene, vinylnaphthalene, or anyvinylaromatic monomer resulting from a C₉ fraction (or more generallyfrom a C₈ to C₁₀ fraction) are suitable, for example, as vinylaromaticmonomer. Preferably, the vinylaromatic compound is styrene or avinylaromatic monomer resulting from a C₉ fraction (or more generallyfrom a C₈ to C₁₀ fraction). Preferably, the vinylaromatic compound isthe minor monomer, expressed as molar fraction, in the copolymer underconsideration.

According to a more particularly preferred embodiment, the plasticizinghydrocarbon resin is chosen from the group consisting of (D)CPDhomopolymer resins, (D)CPD/styrene copolymer resins, polylimoneneresins, limonene/styrene copolymer resins, limonene/(D) CPD copolymerresins, C₅ fraction/styrene copolymer resins, C₅ fraction/C₉ fractioncopolymer resins and the blends of these resins.

The preferred resins above are well known to a person skilled in the artand are commercially available, for example sold, as regards the:

-   -   polylimonene resins: by DRT under the name “Dercolyte L120”        (Mn=625 g/mol; Mw=1010 g/mol; PI=1.6; Tg=72° C.) or by Arizona        Chemical Company under the name “Sylvagum TR7125C” (Mn=630        g/mol; Mw=950 g/mol; PI=1.5; Tg=70° C.);    -   C₅ fraction/vinylaromatic, in particular C₅ fraction/styrene or        C₅ fraction/C₉ fraction, copolymer resins: by Neville Chemical        Company under the names “Super Nevtac 78”, “Super Nevtac 85” or        “Super Nevtac 99”, by Goodyear Chemicals under the name        “Wingtack Extra”, by Kolon under the names “Hikorez T1095” and        “Hikorez T1100”, or by Exxon under the names “Escorez 2101” and        “ECR 373”;    -   limonene/styrene copolymer resins: by DRT under the name        “Dercolyte TS 105” or by Arizona Chemical Company under the        names “ZT115LT” and “ZT5100”.

The level of hydrocarbon resin is preferably between 5 and 60 pce. Belowthe minimum indicated, the targeted technical effect may prove to beinadequate while, above 60 pce, the tackiness of the compositions in theraw state, with regard to the mixing devices, can in some cases becometotally unacceptable from the industrial viewpoint. For these reasons,the level of hydrocarbon resin is more preferably between 5 and 40 pce,more preferably still between 10 and 30 pce.

II-3-B.—Tri- or Pyromellitate Ester

The tri- or pyromellitate ester of the plasticizing system of theinvention corresponds to the formula (I):

in which:

-   -   the R radicals, which are identical or different, represent any        hydrocarbon radical (or chain) which preferably has from 1 to 30        carbon atoms and which can comprise a heteroatom chosen in        particular from S, O and N;    -   the R′ radical represents hydrogen (case of a trimellitate) or        the COOR group (case of a pyromellitate).

Tri- and pyromellitate esters of formula (I) are known as plasticizersfor plastics and various other polymers. They have been described inparticular in the article “Oils, Plasticizers and Other RubberChemicals” (chapter 8, section 8.2, pages 132-137, of Basic RubberTesting (2003), Ed. Dick, John S., Publisher—ISBN: 0-8031-3358-8), andhave also been described, indeed even simply mentioned among otherpossible plasticizing agents, in Patent Applications U.S. Pat. No.4,287,928, EP-A-1 632 364 and WO-A-02/088238, all relating to tyrerubber compositions.

However, to the knowledge of the Applicant Companies, they have neverbeen used in combination with a plasticizing hydrocarbon resin, inparticular in such rubber compositions.

Preferably, the R radicals are chosen from the group consisting oflinear, branched or cyclic alkyls comprising from 1 to 30 carbon atomsand aryls, aralkyls or alkaryls comprising from 6 to 30 carbon atoms.

More preferably, the R radicals represent a linear, branched or cyclicalkyl group comprising from 1 to 20, more preferably still from 1 to 15,carbon atoms.

Mention may be made, as examples of such preferred R radicals comprisingfrom 1 to 15 carbon atoms, for example of the methyl, ethyl,butoxyethyl, butoxyethoxyethyl, propyl, propenyl, butyl, isobutyl,dibutyl, diisobutyl, benzylbutyl, heptyl, hexyl, ethylhexyl, octyl,isooctyl, benzyloctyl, dioctyl, diisooctyl, nonyl, isononyl, ethylnonyl,isodecyl, diisodecyl, tridecyl or octyldecyl radicals.

Use is preferably made of a trimellitate of formula (II):

in which the R radicals, which are identical or different, represent ahydrocarbon radical having from 1 to 20, more preferably from 1 to 15,carbon atoms.

The corresponding preferred molecular weights are typically includedwithin a range from 252 to 1051 g/mol, more preferably from 252 to 841g/mol.

Such preferred trimellitates are commercially available; they have beendeveloped essentially for the plasticizing of rigid plastics, such asPVC. Mention may be made, by way of examples, of the trimellitates ofthe “Jayflex” series sold by Exxon Mobil, in particular thetrimellitates “Jayflex TIOTM” (R═C₈H₁₇ in the formula II) or “JayflexTINTM” (R═C₉H₁₉ in the formula II).

Mention may be made, as other examples of these trimellitates of formula(II), of the plasticizers of the “Palatinol” series from BASF, forexample “TOTM” (R═C₈H₁₇), “TNTM” (R═C₉H₁₉) or “79TM-I” (triethylnonyl),“Plasthall” from CP HALL or “Diplast” from Lonza.

Use is very particularly made of trioctyl trimellitate (abbreviated to“TOTM” or tri(2-ethylhexyl)trimellitate) or triisooctyl trimellitate(abbreviated to “TIOTM”), both of formula (II) in which R represents theC₈H₁₇ group, or a mixture of these two compounds TOTM and TIOTM.

In the rubber composition of the invention, the level of tri- orpyromellitate is preferably between 5 and 60 pce. Below the minimumindicated, the targeted technical effect may prove to be inadequatewhile, above 60 pce, there is a risk of a reduction in grip of the tyreswhen the compositions of the invention are used in the treads of thesetyres. For these reasons, the level of tri- or pyromellitates is morepreferably between 5 and 40 pce, more preferably still between 10 and 30pce.

With regard to the overall level of plasticizing system according to theinvention in the rubber composition of the invention, it is preferablybetween 10 and 100 pce, more preferably between 20 and 80 pce (inparticular between 20 and 50 pce).

All the tri- or pyromellitates described in the present section areliquid at ambient temperature (23° C.). They exhibit a Tg typically ofless than −80° C. Therefore, according to a specific embodiment of theinvention, they could be used, in all or part, as extending oil for thediene elastomers present in the rubber composition of the invention.

II-4. Various Additives

The rubber compositions in accordance with the invention can alsocomprise all or a portion of the usual additives generally used inelastomer compositions intended for the manufacture of tyres orsemi-finished products for tyres, such as, for example, otherplasticizing agents (other than the plasticizing system of theinvention), preferably non-aromatic or very slightly aromaticplasticizing agents, for example naphthenic or paraffinic oils, IVIES orTDAE oils, glycerol esters (in particular trioleates), especiallynatural esters, such as rapeseed or sunflower vegetable oils, pigments,protection agents, such as antiozone waxes, chemical antiozonants,antioxidants, antifatigue agents, reinforcing resins, methyleneacceptors (for example, phenolic novolak resin) or methylene donors (forexample, HMT or H3M), a crosslinking system based either on sulphur oron sulphur donors and/or on peroxide and/or on bismaleimides,vulcanization accelerators, vulcanization activators or antireversionagents.

These compositions can also comprise, in addition to coupling agents,coupling activators, agents for covering the inorganic fillers or moregenerally processing aids capable, in a known way, by virtue of animprovement in the dispersion of the filler in the rubber matrix and ofa lowering in the viscosity of the compositions, of improving theirability to be processed in the raw state, these agents being, forexample, hydrolysable silanes, such as alkylalkoxysilanes, polyols,polyethers, primary, secondary or tertiary amines or hydroxylated orhydrolysable polyorganosiloxanes.

II-5. Manufacture of the Rubber Compositions

The compositions are manufactured in appropriate mixers using twosuccessive preparation phases well known to a person skilled in the art:a first phase of thermomechanical working or kneading (“non-productive”phase) at high temperature, up to a maximum temperature of between 110°C. and 190° C., preferably between 130° C. and 180° C., followed by asecond phase of mechanical working (“productive” phase) up to a lowertemperature, typically of less than 110° C., for example between 40° C.and 100° C., finishing phase during which the crosslinking system isincorporated.

The process in accordance with the invention for preparing a rubbercomposition exhibiting in particular an improved wear resistancecomprises the following stages:

-   -   incorporating in a diene elastomer, during a first stage        (“non-productive” stage), at least one reinforcing filler and        one plasticizing system, everything being kneaded        thermomechanically, in one or more goes, until a maximum        temperature of between 110° C. and 190° C. is reached;    -   cooling the combined mixture to a temperature of less than 100°        C.;    -   subsequently incorporating, during a second stage (“productive”        stage), a crosslinking system;    -   kneading everything up to a maximum temperature of less than        110° C.,        and it is characterized in that the said plasticizing system        comprises, in combination, a plasticizing hydrocarbon resin, the        Tg of which is greater than 0° C., and a tri- or pyromellitate        corresponding to the abovementioned formula (I), preferably a        trimellitate corresponding to the abovementioned formula (II) in        which the R radicals, which are identical or different,        represent a hydrocarbon radical having from 1 to 20 carbon        atoms.

By way of example, the non-productive phase is carried out in a singlethermomechanical stage during which, in a first step, all the necessarybase constituents (diene elastomer, reinforcing filler and couplingagent, if necessary, plasticizing system) are introduced into anappropriate mixer, such as a normal internal mixer, followed, in asecond step, for example after kneading for one to two minutes, by theother additives, optional additional covering agents or processing aids,with the exception of the crosslinking system. After cooling the mixturethus obtained, the crosslinking system is then incorporated in anexternal mixer, such as an open mill, maintained at a low temperature(for example, between 40° C. and 100° C.). The combined mixture is thenmixed (productive phase) for a few minutes, for example between 2 and 15min.

The crosslinking system is preferably a vulcanization system based onsulphur and on an accelerator. Use may be made of any compound capableof acting as accelerator of the vulcanization of diene elastomers in thepresence of sulphur, in particular those chosen from the groupconsisting of 2-mercaptobenzothiazyl disulphide (abbreviated to “METS”),N-cyclohexyl-2-benzothiazolesulphenamide (abbreviated to “CBS”),N,N-dicyclohexyl-2-benzothiazolesulphenamide (abbreviated to “DCBS”),N-tert-butyl-2-benzothiazolesulphenamide (abbreviated to “TBBS”),N-tert-butyl-2-benzothiazolesulphenimide (abbreviated to “TBSI”) and themixtures of these compounds. Preferably, a primary accelerator of thesulphenamide type is used.

Additional to this vulcanization system are various known secondaryaccelerators or vulcanization activators, such as zinc oxide, stearicacid, guanidine derivatives (in particular diphenylguanidine), and thelike, incorporated during the first non-productive phase and/or duringthe productive phase. In the case of use of the composition of theinvention as tyre tread, the level of sulphur is, for example, between0.5 and 3.0 pce and that of the primary accelerator is between 0.5 and5.0 pce.

The final composition thus obtained is subsequently calendered, forexample in the form of a sheet or of a plaque, in particular forcharacterization in the laboratory, or else extruded in the form of arubber profiled element which can be used, for example, as a tyre treadfor a passenger vehicle.

The vulcanization (or curing) is carried out in a known way at atemperature generally of between 130° C. and 200° C. for a sufficienttime which can vary, for example, between 5 and 90 min depending inparticular on the curing temperature, the vulcanization system adoptedand the vulcanization kinetics of the composition under consideration.

The invention relaters to the rubber compositions described above bothin the “raw” state (i.e., before curing) and in the “cured” orvulcanized state (i.e. after vulcanization).

III. EXAMPLES OF THE IMPLEMENTATION OF THE INVENTION

III-1. Preparation of the Rubber Compositions and Treads

The tests which follow are carried out in the following way: thereinforcing filler, the coupling agent, the plasticizing system, thediene elastomer and the various other ingredients, with the exception ofthe vulcanization system, are successively introduced into an internalmixer, 70% filled and having an initial vessel temperature ofapproximately 60° C. Thermomechanical working (non-productive phase) isthen carried out in one stage, which lasts in total approximately from 3to 4 minutes, until a maximum “dropping” temperature of 165° C. isreached.

The mixture thus obtained is recovered and cooled and then sulphur andan accelerator of sulphenamide type are incorporated on an externalmixer (homofinisher) at 30° C., the combined mixture being mixed(productive phase) for an appropriate time (for example, between 5 and12 min).

The compositions thus obtained are subsequently calendered, either inthe form of plaques (thickness of 2 to 3 mm) or of fine sheets ofrubber, for the measurement of their physical or mechanical properties,or extruded in the form of treads for passenger vehicle tyres.

III-2. Rubber Tests and Tyre Running Tests

The aim of these tests is to demonstrate the improved performance of arubber composition according to the invention, in comparison with acontrol composition of the prior art.

For this, two compositions, denoted C-1 and C-2, based on dieneelastomers (SSBR and BR blends) reinforced with silica and carbon blackare prepared. The two compositions are prepared in a mixer which issufficiently big to make possible the manufacture of treads and theperformance of running tests on tyres comprising these treads.

The two compositions tested are identical, except for the plasticizingsystem used, which comprises, in combination, one and the sameplasticizing hydrocarbon resin (polylimonene) as first plasticizingagent and two other types of compound as second plasticizing agent:

-   -   composition C-1: polylimonene resin+MES oil;    -   composition C-2: polylimonene resin+trimellitate.

As explained in the introduction of the present document, compositionC-1 is a reference composition for the Applicant Companies which hasfurtheimore given proof of its excellent performance in terms of wear orabrasion resistance. The MES (for “Medium Extracted Solvates”) oil is anoil of the “non-aromatic” type which is characterized by a very lowlevel of polyaromatics (approximately 20 to 50 times less), incomparison with conventional petroleum-derived aromatic oils whichcomprise a high level of aromatics, known under the name of DAE (for“Distillate Aromatic Extracts”) oils.

Composition C-2, comprising the plasticizing system in accordance withthe invention, is thus itself in accordance with the invention.

Tables 1 and 2 give the make-up of the two compositions (Table 1—levelof the various products expressed in “pce” or parts by weight per onehundred parts of elastomer(s)) and their properties before and aftercuring (30 min at 150° C.); the vulcanization system is composed ofsulphur and sulphenamide.

The examination of the various results in Table 2 shows that the rubberproperties measured are substantially equivalent from one composition tothe other, before and after curing.

Nothing in these rubber properties thus allowed a person skilled in theart to anticipate the improved performance which was observed for thecomposition of the invention, as is explained in detail below.

The two compositions were tested as treads of radial carcass passengervehicle tyres, with a size of 195/65 R15 (speed rating H),conventionally manufactured and in all respects identical apart from theconstituent rubber composition of the tread: composition C-1 for thecontrol tyres (denoted T-1) and composition C-2 for the tyres of theinvention (denoted T-2).

The tyres were tested in accordance with the instructions in thepreceding section 1-6 (“Renault Laguna” vehicle for the grip test;“Mercedes C200” vehicle for the wear resistance test).

The rolling results obtained are given in the appended Table 3.

They clearly demonstrate that the T-2 tyres of the invention offer thebest compromise in properties with very particularly a significantlyincreased (+7%) wear resistance with respect to the T-1 tyres and to theplasticizing system (polylimonene+MES oil), which neverthelessconstitutes a reference with regard to the criterion of wear resistance(see abovementioned Application WO-A-2005/087859). It should beemphasized that this improvement is obtained without having adetrimental affect on the other properties, in particular that of wetgrip.

Other control tyres (T-3) were prepared using, in combination, apolylimonene resin and another plasticizing agent of the family of theester plasticizing agents, in this case an alkyl oleate (“Plasthall7049” from CP HALL) as described, for example, in the abovementionedApplication WO-A02/088238.

The wear resistance test demonstrated a wear resistance for the T-2tyres in accordance with the invention greater by 17% in comparison withthe T-3 tyres, which altogether confirms the superiority of thetrimellitate plasticizing agent over another plasticizing agent of estertype in the presence of one and the same plasticizing hydrocarbon resin.

In conclusion, the novel plasticizing system of the invention offerstyre rubber compositions a particular advantageous compromise inproperties, with an improved wear resistance without having adetrimental affect on the other properties, in particular that of wetgrip.

TABLE 1 Composition No: C-1 C-2 SBR (1) 70 70 BR (2) 30 30 Silica (3) 8080 Coupling agent (4) 6.4 6.4 Carbon black (5) 6 6 Plasticizing agent(6) 15 15 Plasticizing agent (7) 14 — Plasticizing agent (8) — 14 DPG(9) 1.5 1.5 ZnO 2.5 2.5 Stearic acid 2 2 Antiozone wax 1.5 1.5Antioxidant (10) 1.9 1.9 Sulphur 1.1 1.1 Accelerator (11) 2.0 2.0 (1)SSBR with 25% of styrene, 64% of 1,2-polybutadiene units and 25% oftrans-1,4-polybutadiene units (Tg = −18° C.); (2) BR with 4.3% of 1,2-;2.7% of trans-1,4-; 93% of cis-1,4- (Tg = −106° C.); (3) Silica “Zeosil1165MP” from Rhodia, “HDS” type (BET and CTAB: approximately 160 m²/g);(4) Coupling agent TESPT (“Si69” from Degussa); (5) Carbon black N234(ASTM grade); (6) Polylimonene resin (“Dercolyte L120” from DRT); (7)MES oil (Catenex SNR from Shell); (8) Trioctyl trimellitate (“DiplastTM/ST” from Lonza); (9) Diphenylguanidine (Perkacit DPG from Flexsys);(10) N-(1,3-Dimethylbutyl)-N-phenyl-para-phenylenediamine (Santoflex6-PPD from Flexsys); (11) CBS (Santocure from Flexsys).

TABLE 2 Composition No.: C-1 C-2 Properties before curing: Mooney (UM)80 74 t_(i) (min) 6.6 7.0 t₉₀ (min) 26.8 27.2 t₉₀-t_(i) (min) 20.2 20.2K (min⁻¹) 0.113 0.114 Properties after curing: Shore A hardness 68.668.4  M10 (MPa) 6.1 5.9 M100 (MPa) 2.0 2.1 M300 (MPa) 2.5 2.6 M300/M1001.25 1.24 tan(δ)max (40° C.) 0.291 0.280 Breaking stress (MPa) 20.3 20.0Elongation at break (%) 477 486

TABLE 3 Properties (in relative units) T-1 T-2 Rolling resistance 100102 Wear resistance 100 107 Wet grip 100 100 (a value of greater than100 indicates an improved performance with respect to the control T-1 ofbase 100)

1. Rubber composition comprising at least one diene elastomer, onereinforcing filler and one plasticizing system, wherein the saidplasticizing system comprises: a plasticizing hydrocarbon resin, theglass transition temperature (Tg) of which is greater than 0° C.; and atri- or pyromellitate ester corresponding to the formula (I):

in which the R radicals, which are identical or different, represent ahydrocarbon radical and the R′ radical represents hydrogen or the COORgroup.
 2. Rubber composition according to claim 1, in which the Rradicals, which are identical or different, are chosen from the groupconsisting of linear, branched or cyclic alkyls comprising from 1 to 30carbon atoms and aryls, aralkyls or alkaryls comprising from 6 to 30carbon atoms.
 3. Rubber composition according to claim 2, in which the Rradicals represent a linear, branched or cyclic alkyl group comprisingfrom 1 to 20 carbon atoms.
 4. Rubber composition according to claim 3,in which the ester is a trimellitate of formula (II):


5. Rubber composition according to claim 4, in which the trimellitate ischosen from the group consisting of trioctyl trimellitate (TOTM),triisooctyl trimellitate (TIOTM) and the mixtures of these compounds. 6.Rubber composition according to claim 1, in which the level of tri- orpyromellitate ester is between 5 and 60 pce.
 7. Composition according toclaim 1, in which the Tg of the hydrocarbon resin is greater than +20°C.
 8. Composition according to claim 1, in which the number-averagemolecular weight of the hydrocarbon resin is between 400 and 2000 g/mol.9. Composition according to claim 1, in which the polydispersity indexof the hydrocarbon carbon is less than
 3. 10. Composition according toclaim 1, in which the plasticizing hydrocarbon resin is chosen from thegroup consisting of cyclopentadiene (abbreviated to CPD) ordicyclopentadiene (abbreviated to DCPD) homopolymer or copolymer resins,terpene homopolymer or copolymer resins, C₅ fraction homopolymer orcopolymer resins and the blends of these resins.
 11. Compositionaccording to claim 10, in which the plasticizing hydrocarbon resin ischosen from the group consisting of (D)CPD homopolymer resins,(D)CPD/styrene copolymer resins, polylimonene resins, limonene/styrenecopolymer resins, limonene/(D)CPD copolymer resins, C₅ fraction/styrenecopolymer resins, C₅ fraction/C₉ fraction copolymer resins and theblends of these resins.
 12. Composition according to claim 1, in whichthe level of plasticizing hydrocarbon resin is between 5 and 60 pce. 13.Composition according to claim 1, in which the diene elastomer is chosenfrom the group consisting of polybutadienes, synthetic polyisoprenes,natural rubber, butadiene copolymers, isoprene copolymers and the blendsof these elastomers.
 14. Use of a rubber composition according to claim1 for the manufacture of a tyre or semi-finished product made of rubber.15. Tyre tread comprising a rubber composition according to claim
 1. 16.Tyre comprising a tread according to claim
 15. 17. Process for preparinga rubber composition comprising at least one diene elastomer, onereinforcing filler and one plasticizing system, the said processcomprising the following stages: incorporating in a diene elastomer, ina mixer: a reinforcing filler; a plasticizing system; everything beingkneaded thermomechanically, in one or more goes, until a maximumtemperature of between 110° C. and 190° C. is reached; cooling thecombined mixture to a temperature of less than 100° C.; subsequentlyincorporating: a crosslinking system; kneading everything up to amaximum temperature of less than 110° C.; extruding or calendering therubber composition thus obtained; and wherein the plasticizing systemcomprises at least: one plasticizing hydrocarbon resin, the glasstransition temperature (Tg) of which is greater than 0° C.; and a tri-or pyromellitate ester corresponding to the formula (I):

in which the R radicals, which are identical or different, represent ahydrocarbon radical and the R′ radical represents hydrogen or the COORgroup.
 18. Plasticizing system which can be used for the plasticizing ofa diene rubber composition, the said system comprising, in combination:a plasticizing hydrocarbon resin, the glass transition temperature (Tg)of which is greater than 0° C.; and a tri- or pyromellitate estercorresponding to the formula (I):

in which the R radicals, which are identical or different, represent ahydrocarbon radical and the R′ radical represents hydrogen or the COORgroup.
 19. Use of a plasticizing system according to claim 18 for theplasticizing of a tyre diene rubber composition.